1. Field of the Invention
The present invention relates to a projection optical system.
2. Brief Description of Related Art
Lithographic processes are commonly used in the manufacture of semiconductor elements, such as integrated circuits (ICs), LSIs, liquid crystal elements, micropatterned members and micromechanical components.
A projection exposure apparatus used for photolithography generally comprises an illumination optical system having a light source and a projection optical system. Light from the illumination optical system illuminates a reticle (a first object) having a given pattern, and the projection optical system transfers an image of the reticle pattern, onto a region of a photo-sensitive substrate (a second object). The image of the reticle pattern may also be reduced in size by the projection optical system so as to produce a smaller image of the reticle pattern on the substrate.
The trend to ever smaller and more sophisticated miniaturized devices places increasingly high demands on the projection exposure systems and thus projection optical systems used for the manufacture of these devices. In order to achieve higher resolutions in the exposure of substrates, the imaging of the reticle onto the substrate has to be performed with a sufficiently high numerical aperture (NA) on the side of the substrate. Therefore, an increase of the numerical aperture is a decisive factor in the development of improved projection exposure systems.
High numerical apertures bring about a whole range of challenges in terms of a design of a projection optical system. In purely refractive optical systems for projection exposure the requirements for correction of imaging errors such as aberrations and the like are increasing with increasing numerical aperture. Projection optical systems tend to increase in weight and size as the numerical aperture of such systems increases. In particular, diameters of lenses increase to such an extent that they become very expensive and difficult to manufacture, and the manufacture of aspherical lenses of a sufficient accuracy represents a particular problem.